1. Field of the Invention
The present invention relates to a vertical field effect transistor having an active region, which is made of a bundle of linear structures such as nanowires or carbon nanotubes that function as a channel region. The present invention also relates to a method for fabricating a vertical field effect transistor, which can provide nanowires on a region to be an active region such that the nanowires and the channel region are self-aligned with each other.
2. Description of the Related Art
Transistors used in large-scale integrated circuits (LSIs) and thin-film transistors (TFTs) used in flat panel displays are all classified as field effect transistors (FETs) according to their operating principles. And the performance of those transistors has been enhanced by reducing their dimensions. For example, in a silicon semiconductor process, a fine-line patterning process with a design rule of 0.1 μm or less is already realized by shortening the wavelength of an exposing radiation source for use in a photolithographic process.
However, such a size reduction realized by modifying photolithographic process parameters is going to reach a limit. In addition, the smaller the minimum patterning size, the higher the prices of exposure systems and photomasks tend to be.
In view of these considerations, more and more people have attempted to improve the performance of FETs recently by adopting either a totally new material such as strained silicon or germanium (see K. Rim, et al., “Fabrication and Mobility Characteristics of Ultra-thin Strained Si Directly on Insulator (SSDOI) MOSFETs”, IEEE IEDM 2003, p. 49, for example) or a novel structure such as Fin FET (see Y. K. Choi, et al., “Reliability Study of CMOS Fin FET,” IEEE IEDM 2003, p. 177). Among other things, technologies of fabricating a transistor using linear structures such as carbon nanotubes (CNTs) or semiconductor nanowires have attracted much attention in the art. The CNTs and nanowires have a columnar structure with a diameter of just several nanometers and could realize transistors on a nanometer scale. R. Martel, et al. reported a room temperature operation of a transistor using CNTs at a normal temperature in “Single- and Multi-Wall Carbon Nanotube Field-Effect Transistors,” Appl. Phys. Lett. 73, p. 2447, 1998. Also, D. Wang, et al. disclosed a room temperature operation of a transistor using nanowires in “Germanium Nanowire Field-Effect Transistors with SiO2 and High-K HfO2 Gate Dielectrics,” Appl. Phys. Lett. 83, p. 2432, 2003. In the transistors disclosed by Martel et al. and Wang et al., however, their channel length is defined by the photolithographic technique adopted. Accordingly, methods for forming nanometer scale FETs by a self-organizing technique, not by the photolithographic process, have been researched.
A transistor, formed by growing the CNTs or nanowires vertically, is disclosed in U.S. Pat. No. 6,740,910 B2. This is a vertical field effect transistor, in which a CNT is grown in each of a plurality of through holes that are provided through an insulating film and used as a channel region.
It is known that the conductivity of a CNT changes depending how to wind a graphene sheet. In the current CNT growing method, nanotubes with different degrees of conductivity are formed at random. Thus, it is difficult to selectively form nanotubes with a desired degree of conductivity.
As for nanowires on the other hand, a desired degree of conductivity is achieved by selecting an appropriate material. Also, nanowires can be doped with a dopant either by a conventional ion implantation process or by an in-situ doping process to be carried out while the nanowires are growing.
In this manner, the conductivity and doping level are easily controllable by using nanowires. Thus, by introducing the nanowires into a device and by establishing a self-organizing process, a high-performance device could be fabricated at a reduced cost in the near future without complicating the manufacturing process excessively.
According to the conventional techniques disclosed in the documents mentioned above, however, it is difficult to control the direction and positions of nanowires growing.
As for CNTs, there is a report that the growth direction thereof is controllable by utilizing an electric field or a magnetic field. Nevertheless, that method provides a narrow control range, requires a complicated manufacturing process and therefore, cannot be used effectively to make LSIs or TFTs. Regarding the positional control of nanowires, manipulation using STM or AFM and a catalyst position control by a photolithographic technique have been reported. In the positional control process by manipulation, however, each device unit needs to be shifted. Accordingly, this technique cannot be applied effectively to mass-producing large-scale circuits or circuits including multiple types of elements. According to the positional control by the photolithographic technique on the other hand, it is difficult to reduce the dimensions below the exposure limit. For that reason, this technique is not suitable to forming a nanometer scale device.
Also, it is very difficult to grow nanowires like a blanket and then selectively remove unnecessary ones of the nanowires. This is because if a resist pattern were provided on a layer including bundles of nanowires, then the resist material would enter the gaps between the nanowires easily.
Furthermore, in the manufacturing process disclosed in U.S. Pat. No. 6,740,910, the shape and position of each channel region are defined by the shape and position of its associated through hole in the insulating film. Accordingly, to make a very small channel region, an equally small through hole needs to be provided through the insulating film. Thus, the dimensions of a transistor cannot be reduced below the limit of the photolithographic process.